Ukraine braced for war Monday as its president warned of a potential Russian invasion in the wake of Sunday’s attack on three Ukrainian military ships.

President Petro Poroshenko told the Ukrainian Parliament that intelligence pointed to “a highly serious threat of a ground operation against Ukraine.”

He didn’t elaborate while asking the Supreme Rada to impose martial law for 30 days, but insisted that his request “doesn’t mean declaring a war.”

“It is introduced with the sole purpose of boosting Ukraine’s defense in the light of a growing aggression from Russia,” he said in the capital city of Kiev.

The escalating crisis came amid international outrage over the incident in the Kerch Strait, with European Council President Donald Tusk condemning Russia’s “use of force” and demanding it “return Ukrainian sailors, vessels and refrain from further provocations….”

Following an emergency meeting with Ukrainian ambassadors in Brussels, NATO Secretary-General Jens Stoltenberg also repeatedly warned that “Russia has to understand that its actions have consequences.”

Ukraine’s navy has said Russia captured 24 crew members, including six who were wounded when their ships came under fire off Crimea.

Russia invaded Crimea and annexed it from Ukraine, a former Soviet republic, in 2014 and about 10,000 people have been killed in ongoing fighting between Ukrainian forces and Russian-backed separatists in eastern Ukraine since then.

During an emergency meeting of the UN Security Council, US Ambassador Nikki Haley accused Russia of committing “outlaw actions” that were making a “normal relationship” between Washington and Moscow “impossible.” …

…President Trump is scheduled to meet with Russian President Vladimir Putin this week at the G-20 summit in Buenos Aires, Argentina.

Russian Foreign Minister Sergey Lavrov accused Ukraine on Monday of violating international law by failing to get permission before its ships headed into the Kerch Strait, which connects the Black Sea with the Sea of Azov. …

A 2003 treaty [signed by Putin] designates both the Kerch Strait and the Sea of Azov as territorial waters shared by Russia and Ukraine.

But Russia has sought to assert greater control over the narrow passage since seizing Crimea, and this year opened a 12-mile bridge that links the former Ukrainian peninsula with Russia. …..

Poroshenko said 10 of Ukraine’s 27 regions — the ones most likely to bear the brunt of a potential Russian strike — would be subject to martial law.

Areas of martial law in Ukraine.

Details on the extent of the crackdown were unclear on Monday, with measures including a partial mobilization and strengthening of air defenses and vaguely worded plans for stronger anti-terrorism efforts and “information security.”

The move, which was overwhelmingly approved by lawmakers following five hours of debate, marked the first time martial law has been declared in Ukraine.